Filling large gaps between curved geometry using faces?
In case like this it might be easier to continue using curve object for modeling and convert it to mesh once the wave is completed.

Bezier curve objects can be filled to become flat surfaces. It's possible though to do that only when their shape is set to be 2D, i.e. positon of control points is constrained to their local XY axes. You still can rotate curve in Object mode though. 

So grab your curve which was used for wave creating, set its Shape option to 2D and rotate as needed in Object mode. Note - it is important to do that in Object mode as rotating in Edit mode will actually create straight line because movement of control points is constrained by 2 axes.



To give curve thickness use Extrude option in the Properties editor > Object Data tab > Geometry rollout. To control resolution use Preview U option in the Shape rollout.



Once you use everything possible of the curve you can convert it to mesh. Note that most likely you'll need to retopologize the mesh as converted curves often have topology not suitable for anything like later subdivision or beveling.